Internet sharing from scatch

share-from-scratch.md

Internet sharing from scratch on Linux

Something like

steps

Assuming eth0 is the interface which already has internet, and you want to share to eth1,

• Install dnsmasq
• enable ip forwarding sysctl -w net.ipv4.ip_forward=1
• make a short dnsmasq.conf (example below)
• give eth1 an IP, robust to link disconnects with (while true; do ip a add 10.11.0.1/24 dev eth1 &> /dev/null ; sleep 1; done) &
• start dnsmasq, dnsmasq --log-async --keep-in-foreground -z -i eth1 -C dnsmasq.conf -p0 &
• tell iptables to forward on interfaces and masquerade nat
  • iptables -A FORWARD -i eth0 -j ACCEPT
  • iptables -A FORWARD -i eth1 -j ACCEPT
  • iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
• start a tcpdump tcpdump -i eth1

Now, plug/replug $stuff into eth1 and watch what happens. Tcpdump and dnsmasq should show you $stuff getting ip address, and the tcpdump should show traffic making its way in and out.

dnsmasq config

log-queries
log-dhcp
no-daemon
dhcp-broadcast
dhcp-range=10.11.0.2,10.11.0.10
dhcp-option=3,10.11.0.1
dhcp-option=6,1.1.1.1

Replace 1.1.1.1 with your favorite DNS server.